#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
using namespace std;

const int N = 1e6 + 10, INF = 0x3f3f3f3f;

string s;
map<char, int> hs;

void solve()
{
    cin >> s;
    for (int i = 0;i < s.size();i++) {
        hs[s[i]]++;
    }

    int mi = INF;
    char ans;
    for (auto [x, y] : hs) {
        if (y < mi) ans = x, mi = y;
    }
    cout << ans;
}

signed main()
{
    //ios::sync_with_stdio(0);
    //cin.tie(0);cout.tie(0);
#define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    int T = 1;
    //cin>>T;
    for (int i = 1;i <= T;i++) {
        solve();
    }
}